Normal blood coagulation is a complex physiological and biochemical process involving activation of a coagulation factor cascade leading to fibrin formation and platelet aggregation along with local vasoconstriction (reviewed by Davie et al., Biochemistry 30:10363, 1991). The clotting cascade is composed of an “extrinsic” pathway thought to be the primary means of normal coagulation initiation and an “intrinsic” pathway contributing to an expanded coagulation response. The normal response to a bleeding insult involves activation of the extrinsic pathway. Activation of the extrinsic pathway initiates when blood comes in contact with tissue factor (TF), a cofactor for factor VII that becomes exposed or expressed on tissues following insult. TF forms a complex with FVII that facilitates the production of FVIIa. FVIIa then associates with TF to convert FX to the serine protease FXa, which is a critical component of the prothrombinase complex. The conversion of prothrombin to thrombin by the FXa/FVa/calcium/phospholipid complex stimulates the formation of fibrin and activation of platelets, all of which is essential to normal blood clotting. Normal hemostasis is further enhanced by intrinsic pathway factors IXa and VIIIa, which also convert FX to FXa.
Blood clotting is inadequate in bleeding disorders, which may be caused by congenital coagulation disorders, acquired coagulation disorders, or hemorrhagic conditions induced by trauma. Bleeding is one of the most serious and significant manifestations of disease, and may occur from a local site or be generalized. Localized bleeding may be associated with lesions and may be further complicated by a defective haemostatic mechanism. Congenital or acquired deficiencies of any of the coagulation factors may be associated with a hemorrhagic tendency. Congenital coagulation disorders include hemophilia, a recessive X-linked disorder involving a deficiency of coagulation factor VIII (hemophilia A) or factor IX (hemophilia B) and von Willebrands disease, a rare bleeding disorder involving a severe deficiency of von Willebrands factor. Acquired coagulation disorders may arise in individuals without a previous history of bleeding as a result of a disease process. For example, acquired coagulation disorders may be caused by inhibitors or autoimmunity against blood coagulation factors, such as factor VIII, von Willebrand factor, factors IX, V, XI, XII and XIII; or by hemostatic disorders such as caused by liver disease, which may be associated with decreased synthesis of coagulation factors. Coagulation factor deficiencies are typically treated by factor replacement which is expensive, inconvenient (intravenous), and not always effective. As many as 20% of patients receiving chronic factor replacement therapy may generate neutralizing antibodies to replacement factors.
Thus, there remains a need for new therapeutic approaches for treating bleeding disorders. A single pharmaceutical agent that is safe, convenient and effective in a broad range of bleeding disorders would favorably impact clinical practice.